This invention relates to novel polymeric compositions which in one of their embodiments are polymer particles having a plastic core of a fluoropolymer such as polytetrafluoroethylene, poly(vinylidene fluoride), or polychlorotrifluoroethylene, and a shell surrounding the fluoropolymer core of an elastomeric fluoropolymer.
Particulate polymeric compositions having a core of one type of polymeric material and a shell of another type of polymeric material are known and are usually referred to as core/shell or core/sheath polymers. The polymeric materials forming the core and the shell usually have different chemical compositions; for example, they may be polymers of different monomers or copolymers of different proportions of the same monomers; or they may have the same compositions but different molecular weights. The core/shell polymers of the present invention have an elastomeric shell surrounding a crystalline, non-elastomeric core.
It is known that the tear strength of certain fluoroelastomers can be improved by dispersing in the fluoroelastomer matrix particles of polytetrafluoroethylene (PTFE), as taught in Japanese published application No. 56-10895 (1982) of Daikin Kogyo K.K. and as earlier discussed by Kaufman et al. in Rubber Chemistry and Technology, March 1963, pp. 527-532. While the Japanese inventors provide no information on the processability of their PTFE-filled compositions, Kaufman et al. observed that addition of PTFE to fluoroelastomers caused the compositions to lose their workability rapidly. Improvement of the tear strength thus is achieved at the expense of processability, which makes the technique impractical, a least at certain PTFE levels, for example above 10-15 weight percent of PTFE.
It would be very desirable to be able to disperse PTFE or another plastic fluoropolymer in a fluoroelastomer matrix at any desired level to achieve a significant improvement of tear strength without causing such undesirable changes. One possible approach would be to render the plastic fluoropolymer compatible with and readily dispersible in the fluoroelastomer by chemically modifying the polymer molecule.
U.S. Pat. No. 4,326,046 to Miyaka et al. discloses a modified PTFE which is made by first polymerizing tetrafluoroethylene (TFE) in an aqueous medium containing a dispersing agent, then copolymerizing TFE with an acid group-containing fluoromonomer in the presence of the homopolymer made in the first step. The resulting polymeric material has a core/shell structure.
U.S. Pat. No. 4,158,678 to Tatemoto et al. describes segmented polymers, which are made in the presence of an iodine-containing organic compound. The segmented polymers thus are terminated at one end by an iodine atom and at the other end by the remaining radical of the initial iodine-containing organic compound. At least one segment contains fluorine atoms, and the molecular weight of each segment is no less than 10,000, one segment having a molecular weight of at least 30,000. The first segment may be, among others, tetrafluoroethylene or vinylidene fluoride; and other segments may be, for example, a copolymer or vinylidene fluoride and hexafluoropropylene. The adjacent segments of those polymers are bonded to one another.
German published application OS No. 2,949,907 of Hoechst AG describes core/inner shell/outer shell polymers in which the core is a slightly modified TFE polymer; the inner shell is TFE homopolymer; and the outer shell is a TFE copolymer with another fluoromonomer.
The modified polymers of U.S. Pat. No. 4,326,046 are said to be particularly suitable in reinforcing cation exchange resin membranes. The core/shell polymers of the patent are easily fibrilated. The polymers of the German publication are suitable for improving certain paste-extrudable resins. At higher levels they would not function as fluoroelastomer additives to improve tear strength.